1. Field of the Invention
The invention relates to magnetic article surveillance systems in general, and in particular, to harmonic phase comparison signal processing for such systems.
2. Statement of Art
Article surveillance systems using soft magnetic materials and low frequency detection systems have been known since the Picard patent (763,861) was issued in France in 1934. Picard discovered that when a piece of metal is subjected to a sinusoidally varying magnetic field, an induced voltage, characteristic of the metal composition, is produced in a pair of balanced coils in the vicinity of the applied field. Today, such systems utilize the harmonics produced by a marker of soft magnetic strip to detect the marker. Due to the nonlinear characteristics of such markers, groups of even and odd order harmonics can be produced simultaneously or individually. Odd order (1, 3, 5 . . . ) harmonics are produced by a symmetrical switching of the B/H loop. Even order harmonics (2, 4, 6 . . . ) are produced by a non-symmetrical switching condition, typically caused by a D.C. magnetic bias internal or external to the material.
The nonlinear characteristics of the soft magnetic material, while not commonly found, can be duplicated in some ferrous alloys by the presence of a magnetic bias. This results in the generation of even and odd order harmonics that duplicates the response of soft magnetic materials, such as Permalloy and the metallic glass products. However, the use of more sensitive detection equipment can add to the probability of false alarms due to ferrous alloys. More sensitive detection equipment also increases the difficulty of effectively deactivating markers, that is, turning markers "off".
Another limitation of the soft strip and low frequency system is that only a single bit of information is available during marker and system interaction. The marker is either in the detection zone, or not. The only other alternative is that the marker is, whether or not in the detection zone, deactivated. While this is not a disadvantage for systems used in theft control, it is an extreme limitation when used for monitoring the flow of a group of differing objects, or even persons, through the detection zone.
Those systems using coded devices for monitoring people and articles in a selected area are quite capable of a large number of codes. Card access systems are a good example. They generally combine a digital network and/or radio frequency circuit to transmit the code. However these devices are too expensive to use either for theft control of low cost items or for inventory control in factories or stores. It is understood that encoded markers can be affixed to or otherwise carried by any article or person, animal, etc. The term "article" is used herein to encompass such possibilities.
A magnetic article surveillance system disclosed in U.S. Pat. No. 4,622,542 differed from the prior art in that the codes utilized are not duplicated by biased ferrous alloys, even accidentally. Further, the coded marker can be embodied in a single element device and can be programmed (code changed) by altering the geometry of or extent of a conductor surrounding a magnetic core. It is detectable at large distances and is not sensitive to spatial orientation within the system. Article surveillance is based upon the detection of phase shifted harmonics generated by markers in a detection zone. The number of codes does not depend on the marker structure but on the phase resolution of the detection system and programming device.
Such a system, based upon phase shifted harmonics, has already lent itself to an improved system for reliably deactivating magnetic markers, which system is taught in commonly owned, copending application Ser. No. 052,240. As the prior art is explained therein, prior art markers utilizing a single strip of soft magnetic material can be deactivated by placing one or more elements of a high coercivity material along the length of the single strip. A magnetic bias applied to and retained by the high coercivity material reduces the harmonic generation of the single, soft magnetic strip. This technique is often unreliable, and usually is ineffective when the marker encounters the high field intensity of the transmitter and a closely coupled, highly sensitive receiver. Such a technique is also particularly ineffective for preventing false alarms due to the presence of multiple deactivated tags. The deactivation technique of such prior art systems is such that the harmonic signal generated by the soft magnetic material is normally not completely eliminated. Assuming, for example, that a system is effective to reduce the amplitude of the harmonic signal to ten percent (10%) of its normal level, then a consumer carrying ten deactivated tags on ten legitimately purchased articles will likely set off an alarm due to the cumulative amplitudes of the ten damped harmonic signals.
However, the ability to control harmonic phase permits the generation of signals having a unique signature, apart from both ferrous alloys and soft magnetic materials. This avoids the accidental detections plaguing prior art systems as described above. In addition, a number of codes can be established according to the phase shift induced. The phase shift is not affected by a low level, external magnetic bias, in that odd order products are totally unaffected and even products shift by +1 -180 degrees.
In accordance with the teachings of U.S. Pat. No. 4,622,542, a conductive material surrounding a soft magnetic material is responsible for a predetermined phase shift of the harmonic signal generated by the marker in a surveillance or detection zone. A second ferromagnetic element, of higher coercivity than the core material, may be placed over the conductive material. Whenever the higher coercivity magnetic material is itself magnetized by an external magnetic field, the higher coercivity magnetic material has the effect of shutting off that portion of the marker (i.e., the core) so that the harmonic signal is not affected by the conductive material. In effect, the core becomes blind to the presence of the conductive material and the harmonic signal is not phase shifted by the predetermined amount necessary to constitute an alarm condition. Accordingly, the problem of false alarms due to the presence of multiple deactivated tags is eliminated altogether, as the reduced phase shifts of deactivated tags are not cumulative. For example, ten harmonic signals, each of which is shifted by only ten degrees, rather than for example by 100 degrees, will not appear to be cumulatively shifted by 100 degrees.
Development of such deactivatable coded markers brought to light a potential difficulty with multiply coded markers. The phase of odd order marker generated harmonics is dependant upon the saturation characteristics of the soft magnetic material, and the saturation characteristic changes somewhat as the field intensity of the transmitted signal varies. In other words, when a coded marker enters a detection zone, the intensity of the transmitted electromagnetic field tends to vary. The variation in intensity causes the phase of the odd harmonics to shift to some degree. This does not present a problem for basic kinds of markers coded only for on or off theft detection systems, but eventually causes a problem when high resolution is needed in order to decode relatively small phase shifts. It has been discovered that this problem can be solved by incorporating into an article surveillance system, operating on phase shifted harmonics, means for automatically adjusting the transmitted reference signals responsive to a characteristic of the received harmonic signals, for example the intensity, to compensate for the variation due to marker presence in the detection zone and thereby prevent the random variation in the phase shift of the harmonic signals.
Further development of phase detection based systems has also revealed that, under certain circumstances, the phase shift between the signal generated by the phase-locked oscillator (of the transmitting circuit) and the detected harmonic(s) may be affected by the presence in the surveillance zone of large metal objects, such as shopping carts. Such objects can alter the phase of the magnetic flux coupling the transmitter to a surveillance tag and can also effect the phase of the currents produced in the coils of the antenna in the receiving circuit by the tag. The invention of this application is based upon the recognition that immunity to these disturbances can be achieved by measuring the phase between two or more of the harmonic signals returned from the tag, because the effect of such large metal objects in the surveillance zone has been found to be the same on all of the harmonic signals. If a predetermined phase shift from the fundamental frequency f.sub.0 to a first harmonic frequency f.sub.1 is a.degree. and from the fundamental frequency f.sub.0 to a second harmonic frequency f.sub.2 is b.degree.; and f.sub.1 and f.sub.2 are shifted randomly but equally by a large metal object in the surveillance zone, then the phase difference (a.degree.-b.degree.; or, b.degree.-a.degree.) between f.sub.1 and f.sub.2 is a reliable indicator of marker detection.
The use of a harmonic characteristic difference calculation in a surveillance system for a different purpose is disclosed in U.S. Pat. No. 4,489,313 - Pfister. A directional loop antenna array is provided by two flat parallel spaced apart open loops with a shorted turn disposed between the open loops equidistant therefrom. The signals from the open loops are vectorially added and subtracted in a sum and difference circuit and the phase angle between the sum and difference signals is ascertained in a phase detector circuit that feeds an indicator. The so determined phase angle is either greater or less than 90.degree. depending upon the relative magnitudes of the loop signals. Such an arrangement provides an output indicative of the direction to a source of AC signals, and in particular, from an identification tag passing through a portal interrogating station. The system enables a determination to be made as to which direction the tag is travelling through the portal. Except for the singular case where the signal originates from a point lying in the plane that is equidistant from, parallel to, and between the planes of the loops, the signal amplitude induced in the loops will be unequal. It is this in equality that is utilized to determine on which side of the loops the source is located. The signal inequality arises as a consequence of the attenuation of a signal with distance of travel. There is nothing in the Pfister system which suggests the phase determination for detecting a marker as utilized in the present invention, not withstanding the use of a phase measurement.
Reference may be made herein to theories and principles which are more fully explained in patents and other applications commonly owned with this application. Accordingly, the teachings of U.S. Pat. No. 4,622,542, U.S. Pat. No. 4,675,657 and application Ser. No. 052,240 are fully incorporated herein by reference.